target/ppc/spapr_caps: Add support for tristate spapr_capabilities

target/ppc/spapr_caps: Add support for tristate spapr_capabilities

spapr_caps are used to represent the level of support for various
capabilities related to the spapr machine type. Currently there is
only support for boolean capabilities.

Add support for tristate capabilities by implementing their get/set
functions. These capabilities can have the values 0, 1 or 2
corresponding to broken, workaround and fixed.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit 6898aed77f4636c3e77af9c12631f583f22cb5db)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>